i think your numbers may be a little off, i got somewhere between 172 and 178 on the exh before i did any port work. thats why i don't raise the port very much. but then again, these motors are completely different than the next one on the assembly line. did you line up zero with bdc and not tdc, then note where the port is closed and double it?
as far as design is concerned, check out
Gordon Jenning's Archivek you can download it free. there is a formula on page 33 for figuring tuned length based on exh timing, and rpm for max power. the diameter of the port at the exh flange is another important measurement. other than those few things the rest sort of depends on what type of power curve youre after. i highly reccommend leaning towards the enduro side of things since you have a fixed gear. anyways, read the chapter on expansion chambers before, during, and after you build your pipe. i would stay away from the design software because you won't understand why you are using the angles youre using, therefore wont be able to modify or fine tune anything. but, here is an immensely helpful program that you cant do without and is free
Cone Layout - Product